1. Field of the Invention
The present invention relates to a method of measuring a wavelength dispersion amount and an optical transmission system, and in particular to a method of measuring a wavelength dispersion amount when wavelength-division multiplexing (WDM) signals are transmitted through optical transmission lines, and an optical transmission system using such a measuring method.
2. Description of the Related Art
Rapid growth of the Internet has been accompanied by corresponding rapid growth in demand for information transmission; a wavelength-division multiplexing (WDM) optical transmission system has been developed which combines from several tens to several hundreds of wavelengths into a single optical fiber for the transmission, and such a system is rapidly being deployed. At present, for the transmission rate per wavelength, 2.4 Gb/s is mainstream, which is hereafter expected to be replaced by a more rapid 10 Gb/s rate, and systems capable of 40 Gb/s are currently in development.
Several configurations for such a WDM optical transmission system are conceivable; the simplest of these is a point-to-point (P-P) system in which, as shown in FIG. 24A, “n” wavelengths are multiplexed at a transmission station (optical terminal node) TERM1, and attenuated optical signals are amplified by a relay amplifier node ILA while being transmitted, with the multiplexed state unchanged, to a reception station (optical terminal node) TERM2.
Optical transmission systems are also known in which, as in FIG. 24B, an optical add-drop (OADM) node is inserted between the transmission node TERM1 and the reception node TERM2, and a portion of the wavelengths in the wavelength-multiplexed optical signal are inserted or branched by an optical band-pass filter or other wavelength selection means. In this case, processing is performed at the optical level. Therefore, the systems are characterized in that functions of optical/electrical or electrical/optical conversion are not required.
FIG. 24C shows an arrangement of an optical cross-connect (OXC) (in this case, 2×2) in which switching functions per wavelength are realized through optical-level wave demultiplexing/multiplexing, and switching functions. Also in the case of this optical cross-connect, optical/electrical or electrical/optical conversion functions are not required.
Because, as described above, the number of WDM wavelengths has increased to several hundreds of channels, it becomes necessary to prepare transponders (devices to perform optical/electrical conversion, reproduce a signal, perform conversion of a modulation format and a wavelength, and connect to another device) for the number of wavelengths at the transmission and reception nodes, thereby increasing costs.
In consideration of this, a changeover to optical add-drop methods and optical cross-connect methods, which enable flexible switching of optical paths (routes) and which do not require optical/electrical or electrical/optical conversion, is anticipated. Development is currently underway to realize such an optical transmission system at low cost.
In the point-to-point system shown in FIG. 24A, which is currently mainstream of the WDM optical transmission system, if the transmission rate per wavelength increases as described above, a time slot per bit is shortened, and a wavelength bandwidth of a modulation signal spectrum is broadened. Therefore, waveform distortion (pulse broadening) due to wavelength dispersion gives rise to intersymbol interference, and this in turn is a factor limiting transmission lengths.
Consequently, technology at relay amplifier nodes for dispersion compensation of wavelength dispersion accumulated over an optical fiber transmission line has become important, and technology is employed which uses a dispersion compensation fiber (DCF) having a wavelength dispersion amount opposite to that of an optical fiber and performs adjustment such that the total wavelength dispersion amount is zero.
In order to design the wavelength dispersion amount of such a dispersion compensation fiber, it is necessary to know the wavelength dispersion amount in established optical transmission lines.
The most reliable method for obtaining such knowledge is to measure the actual wavelength dispersion characteristic. For example, methods which have been proposed are as follows: (1) a method of obtaining the characteristic by measuring the wavelength dependency of a transmission delay time of an optical pulse using a variable-wavelength light source, and by differentiating the wavelength dependency using the wavelength, as disclosed in Japanese Patent Application Laid-open No.8-334436, and (2) a method of obtaining the characteristic by measuring the component reflected by Rayleigh backscattering partway through an optical transmission line, using OTDR (Optical Time Domain Reflectometry), as disclosed in Japanese Patent Application Laid-open No.8-5515.
In cases where measurement of the actual wavelength dispersion is not possible, a method is adopted in which the transmission length is computed from a measured value of the loss in the established optical transmission line, and the wavelength dispersion amount of the established optical transmission line is estimated from catalog values of optical fiber parameters or measured values for optical fibers with similar characteristics.
In this way, by either measuring in advance the actual wavelength dispersion amount in the optical transmission line in use, or by estimating the wavelength dispersion amount from the transmission line length and a span loss amount based on the optical fiber specifications, the wavelength dispersion amount required for the dispersion compensation fiber is generally determined. However, in the cases of the optical add-drop method shown in FIG. 24B, and in the optical cross-connect method shown in FIG. 24C, it is anticipated that application of a technique like those described above will be difficult, for the following reasons:
In an optical network having optical cross-connect functions, for example, generally if there are N nodes, there exist N×N logical paths, as is seen from FIG. 24C. Furthermore, if a plurality of paths (routes) are conceivable to reach the same node, there exist an even greater number of path combinations.
In order to perform transmission without causing faults even when switching paths in the midst of such a large number of paths, it is safest to perform complete dispersion compensation for all paths in advance. This, however, limits the freedom of switching configurations; and because major changes in optical fiber characteristics due to path switching are conceivable, as for example when changing from single-mode fiber (SMF) to nonzero dispersion shifted fiber (NZDSF), it is difficult to adjust the wavelength dispersion amount of all paths to zero.
Further, it is unrealistic to attempt to measure wavelength dispersion amount in advance for all of a huge number of paths; and in cases where new paths are added, measurements for all the newly added paths must be performed once again. In such cases there is a problem that lines in use must be temporarily interrupted.
Also, a method in which a single dispersion compensation fiber is used to compensate for the dispersion amount of all WDM wavelengths at once is generally used due to its cost effectiveness. However, since the wavelength dispersion amount of the optical fiber is wavelength-dependent, the dispersion amount increases in propagating from the transmission node TERM1 to the reception node TERM2, as shown in FIGS. 25A and 25B, and optical dispersion compensation amounts differ among channels with the shortest and with the longest wavelengths.
Accordingly, in order to completely compensate for the wavelength dispersion amount, a design must also take into consideration the accumulated shifts in such wavelength dispersion amounts; however, it is extraordinarily difficult to perform complete dispersion compensation for all routes, while also taking wavelength dependency into account.
For the above reasons, there has been a problem that current methods for designing dispersion compensation amount cannot easily be applied to WDM optical transmission systems incorporating the functions of optical add-drop or optical cross-connect methods.